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3. Experimental Methodology

[6] We introduced a 9 g/L TiO₂ suspension 0.7 m upgradient of the leading edge of the vegetation, which was located approximately 10 m from the head of the channel (Figure 1a). The injection was accomplished by using a metering pump to deliver the TiO₂ tracer at constant rate (100 mL/min) through a slotted hose that spanned the central 1.8 m of the channel and rested 24 cm below the water surface (Figure 1b). The duration of the injection was 0.9 hours. Sampling for the TiO₂ began before the start of the injection and continued for 3 hours after its termination. Water samples (20 mL) were repeatedly collected in plastic scintillation vials by applying suction to 1/8" stainless steel sampling tubes installed at discrete points located 6.8 m down channel from the injection (Figures 1a and 1b). The sampling points were designated as LS, LM, LD, CM, RS, RM, and RD, where L (left), C (center), and R (right) delineate lateral position and refer to locations 0.8 m inside the left wall, at the channel center, and 0.8 m inside the right wall, respectively, and S (shallow), M (mid-depth), and D (deep) delineate vertical position and refer to depths of 0.15, 0.27, and 0.42 m, respectively. Concentrations of titanium (Ti) in the surface-water samples were measured in the laboratory by inductively coupled plasma mass spectrometry following acid dissolution of the TiO₂ particles.

Figure 1. (a) Plan-view positions of the tracer-injection source and samplers within the portion of the channel used for the particle-tracer experiment. The model domain is situated near the head of the 100-m long channel. (b) Cross-sectional positions of the tracer-injection source and samplers as viewed from the head of the channel. [larger image]